Home About us Contact
|
Home About us Contact | ||
|
|
||
ESR Spectra (esr + spectrum)
Selected AbstractsEffect of NaCl filler on ferroelectric phase and polaron configurations of PVDF filmsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 4 2007I. S. ElashmawiArticle first published online: 8 MAR 200 Abstract Polyvinyldene fluoride (PVDF) films filled with NaCl of mass fraction range 1 , W , 6 % were prepared by casting technique. Their crystalline structure, thermal, optical properties and Electron spin resonance (ESR) were examined. X-ray diffraction (XRD) and differential thermal analysis (DTA) measurements indicated a maximum ferroelectric ,-phase increment at 4%. DTA was used to identify the phase transition temperatures, the order of reaction and the activation energy of melting. The UV-Visible optical absorption implied a minimum value of the estimated optical energy gap at W = 4%. ESR spectra contained a Lorentizian signal exhibiting a minimum value of the symmetry factor at W = 4%. The energy levels of the optical gap boundaries were though to contribute to ESR transitions. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Detection of diluted marine tertiary tephra by electron spin resonance and magnetic measurementsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2003B. Ananou SUMMARY Oligocene sediments from ODP leg 115 (South Indian Ocean), Site 709 and Site 711, have been investigated using electron spin resonance (ESR) and magnetic susceptibility measurements, to detect volcanic tephra layers of supposed Ethiopian traps origin. The results obtained at room temperature, without separating the volcanic material from the bulk sediment, show that the ash-content strongly influences the lineshape and intensity of the ESR signal. As a result, the ESR alone, can be used as a powerful tool for characterizing the diluted ash-content of marine sediments. We have also found a strong similarity between the ESR spectra of the same tephra layers from the two sites. [source] Electrochemical, ESR and quantum chemical study of 1-substituted naphthalenes and their radical anions,,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2008N. V. Vasilieva Abstract Electrochemical reduction and oxidation of a series of 1-substituted naphthalenes (1-X-naphthalenes) have been studied by the method of cyclic voltammetry (CV). The first reduction peak of the majority of these compounds corresponds to a one-electron transfer to form the relatively stable radical anion (RA). For these species, ESR spectra have been registered and interpreted, the life time has been estimated. The first oxidation peaks of 1-X-naphthalenes are irreversible and correspond to a transfer of two or more electrons. Copyright © 2007 John Wiley & Sons, Ltd. [source] Catalysis in electron transfer reactions: facts and mechanistic insights,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2002Shunichi Fukuzumi Abstract Catalysis in electron transfer reactions between electron donors and acceptors is described and the important mechanistic insight is provided by showing a number of examples of both thermal and photochemical reactions that involve metal ion-catalyzed electron transfer processes as the rate-determining steps. The quantitative measure of Lewis acidity of metal ions was obtained from the gzz values of ESR spectra of superoxide,metal ion complexes which vary significantly depending on the type of metal ions. Copyright © 2002 John Wiley & Sons, Ltd. [source] Search for highly resolved electron spin resonance spectra of the transient radical in radical polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2002Mikiharu Kamachi Abstract The detection of highly resolved spectra in electron spin resonance (ESR) measurements of radical polymerization is presented. Well-resolved ESR spectra of the propagating radical were detected in the radical polymerization of several vinyl monomers with a specially designed cavity and cell. More highly resolved ESR spectra of the propagating radicals of vinyl and diene compounds were observed with aconventional spectrometer without the specially designed cavity and cell. On the basis of the ESR spectra, propagation rate constants and dynamic behavior of propagating radicals are discussed. Moreover, the application of time-resolved ESR spectroscopy to research on the initiation process in radical polymerization is shown. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 269,285, 2002 [source] Effect of hindered piperidine light stabilizer molecular structure and UV absorber addition on the oxidation of HDPE.JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 4 2004Part 2: Mechanistic aspects, electron spin resonance spectroscopy study, molecular modeling The first in this series of papers explored the effect of the structural characteristics of 2,2,6,6-tetramethylpiperidine-based hindered amine light stabilizers (HALS) on the thermal and photostabilization of high-density polyethylene. In the second part, the energies (stabilities) of the nitroxyl radicals and various intermediate species have been predicted using AccuModel® and related to stabilization performance and electron spin resonance (ESR) spectral data. Nitroxyl radicals with low predicted stability generally afforded improved thermal and photostabilization. ESR spectra were used to obtain values of nitroxyl radical concentration ([>NO·]) and g -factor as a function of pre-aging time for combinations of pre-aged HDPE and >NH HALS. Demethylation reactions of > N-methyl HALS resulted in uselessly weak ESR spectra. The HALS that afforded poor thermal oxidative stabilization gave rise to pronounced minima in [>NO·] that coincided with a maximum in hydroperoxide concentration. The g -factor values indicated that a predominant nitroxyl canonical form generally promoted superior thermal oxidative stabilization, whereas a predominance of the dipolar N·+O, form promoted superior photo-oxidative stabilization. These trends may be related to greater radical-scavenging and peroxide-trapping effects, respectively. Molecular modeling and ESR spectra can therefore provide valuable insight into the effectiveness of HALS and stabilization mechanisms. J. Vinyl Addit. Technol. 10:159,167, 2004. © 2004 Society of Plastics Engineers. [source] ESR/DFT study of bis-iminophosphorane cation radicalsMAGNETIC RESONANCE IN CHEMISTRY, Issue 12 2007Adil Matni Abstract Bis-iminophosphoranes containing various types of linkers between two R3PN moieties were electrochemically oxidized at controlled potential in situ in the electron spin resonance (ESR) cavity. For linkers constituted of phenylenes, conjugated phenylenes or merely a dicyanoethylenic bond, this oxidation led to well-resolved ESR spectra which were characterized by their g values and by their 1H, 14N and 31P isotropic hyperfine constants. These coupling constants agree with those calculated by DFT for the corresponding cation radicals. Experimental and theoretical results clearly indicate that in these species the unpaired electron is mostly delocalized on the bridge and on the nitrogen atoms while the spin density on the phosphorus atoms is particularly small. Cyclic voltammetry and ESR spectra show that the nature of the bridge between the two iminophosphoranes considerably influences the oxidation potential of the compound as well as the stability of the radical cation. Information about the conformation of the precursor containing two Ph3PN moieties separated by a C(CN)C(CN)group was obtained from its crystal structure. Copyright © 2007 John Wiley & Sons, Ltd. [source] Defects and structure of µc-SiOx:H deposited by PECVDPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Lihong Xiao Abstract Electronic transport and paramagnetic defects detected by Electron Spin Resonance (ESR) in both intrinsic and ESR studies on superparamagnetic Fe3O4 nanoparticlesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004Y. Köseo Abstract In this study we have investigated temperature and frequency dependence of magnetic properties of superparamagnetic iron oxide nanoparticles (SPION). ESR measurements have been carried out in a temperature range of 10,300 K. A single, relatively broad and temperature dependent EPR signal was observed at all measurement temperatures. The linewidth slightly increases with decreasing temperature down to 100 K, then it sharply increases down to 60 K. Below 60 K the trend is reversed and the linewidth start to decrease. The resonance field remains almost constant down to 100 K and decreases sharply as the temperature is decreased further. The resonance field of the ESR spectra of Fe3O4 shows a linear dependence on microwave frequency. By using experimental results, the effective g-value and internal field are deduced as 1.9846 and ,40 G, respectively. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Magnetic anisotropy studies on FeNiCo/Ta/FeNiCo three layers film by layer sensitive ferromagnetic resonance techniquePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2004F. Yildiz Abstract Two ferromagnetic layers that are separated by a non-magnetic spacer have been deposited in the presence of an external Dc magnetic field that has been applied in a mutually perpendicular direction during the deposition of each layer. This thin three-layer film of FeNiCo/Ta/FeNiCo was then studied by electron spin resonance (ESR) technique for different directions of the external magnetic field with respect to the film plane. Beside very anisotropic and strong two main modes, relatively well-resolved and very weak subsidiary peaks in ESR spectra were observed on low field side of the main modes for almost every direction of the external field. The anisotropic behavior as the field is rotated from the film normal towards to the film plane is due to strong demagnetizing field. However, two fold (axial) small anisotropies having 90 degrees phase shifts with respect to the peaks from two different layers were seen to originate from field-induced anisotropy. The small peaks are attributed to magnetostatics modes as well. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The study of the miscibility and morphology of poly(styrene-co-4-vinylphenol)/poly(propylene carbonate) blendsPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 8 2004Furong Qiu Abstract Blends of poly(propylene carbonate) (PPC) with copolymer poly(styrene-co-4-vinyl phenol) (STVPh) have been studied by electron spin resonance (ESR) spin probe method and Raman spectroscopy. The ESR results indicated that the nitroxide radical existed in a PPC-rich and an STVPh-rich micro domain in the blends, corresponding to the fast-motion and slow-motion component in the ESR spectra, respectively. And in the temperature dependence composite spectra, the fast-motion fraction increased with increasing the hydroxyl group content in copolymer STVPh. Moreover, the ESR parameter T5mT, rotational correlation times (,c) and activation energies (Ea) showed similar dependence on the hydroxyl group content as the fast-motion fraction. It resulted from the enhancement of the hydrogen-bonding interaction between the hydroxyl groups in STVPh and the carboxyl groups and ether oxygen in PPC. However, the distinct band shift and intensity change among the Raman spectra of pure polymer components and those of the blends were observed. In the carboxyl-stretching region, the band shifted to lower frequency with increasing the hydroxyl groups. Furthermore, the phase morphologies of the blends were obtained by optical microscopy. All could be concluded that the hydrogen-bonding interaction between the two components was progressively favorable to the mixing process and was the driving force for the miscibility enhancement in the blends. Copyright © 2004 John Wiley & Sons, Ltd. [source] Supporting mechanism of non-toxic chromium (III) acetate on silica for preparation of Phillips ethylene polymerization catalystsASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Pengyuan Qiu Abstract Phillips catalyst is an important kind of industrial polyethylene catalyst. As early as in the late 1970s, CrO3 was substituted by chromium (III) acetate for the preparation of Phillips catalyst on the industrial scale owing to health and environmental considerations. There is still considerable research focusing on the relations between the preparation process and catalyst properties in academics. In this work, the supporting mechanism of chromium (III) acetate on silica has been studied by Thermogravimetry,Differential Thermal Analysis (TG-DTA), and Electron Spin Resonance (ESR), in comparison with that of supporting CrO3 on SiO2. The basic chromium (III) acetate supported on high surface area silica gel decomposed differently from that for bulk basic chromium acetate when decomposition temperature was decreased by 15 °C. The decomposition temperature was 299 °C for Cr3(OH)2(Ac)7/SiO2 catalyst precursor, which would be firstly transferred into CrO3 followed by supporting on silica surface as chromate species. The further weight loss came from thermal inductive reduction of chromate species into Cr2O3, which was also supported by the results of colors of catalysts. Moreover, with the increase of chromium loading of Cr3(OH)2(Ac)7/SiO2, such thermal inductive reduction became more severe. ESR spectra of Cr3(OH)2(Ac)7/SiO2 and CrO3/SiO2 catalyst precursors showed that a small amount of supported Cr5+ can exist stably on silica gel surface at temperatures higher than 200 °C. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Selective Inclusion of Electron-Donating Molecules into Porphyrin Nanochannels Derived from the Self-Assembly of Saddle-Distorted, Protonated Porphyrins and Photoinduced Electron Transfer from Guest Molecules to Porphyrin DicationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2007Takahiko Kojima Prof. Abstract A doubly protonated hydrochloride salt of a saddle-distorted dodecaphenylporphyrin (H2DPP), [H4DPPP]Cl2, forms a porphyrin nanochannel (PNC). X-ray crystallography was used to determine the structure of the molecule, which revealed the inclusion of guest molecules within the PNC. Electron-donating molecules, such as p -hydroquinone and p -xylene, were selectively included within the PNC in sharp contrast to electron acceptors, such as the corresponding quinones, which were not encapsulated. This result indicates that the PNC can recognize the electronic character and steric hindrance of the guest molecules during the course of inclusion. ESR measurements (photoirradiation at ,>340,nm at room temperature) of the PNC that contains p -hydroquinone, catechol, and tetrafluorohydroquinone guest molecules gave well-resolved signals, which were assigned to cation radicals formed without deprotonation based on results from computer simulations of the ESR spectra and density functional theory (DFT) calculations. The radicals are derived from photoinduced electron transfer from the guest molecules to the singlet state of H4DPP2+. Transient absorption spectroscopy by femtosecond laser flash photolysis allowed us to observe the formation of 1(H4DPP2+)*, which is converted to H4DPP+. by electron transfer from the guest molecules to 1(H4DPP2+)*, followed by fast disproportionation of H4DPP+., and charge recombination to give diamagnetic species and the triplet excited state 3(H4DPP2+)*, respectively. [source] Solid-state ESR differentiation between racemate versus enantiomerCHIRALITY, Issue 4 2006Pavel A. Levkin Abstract Noticeable differences between solid-state ESR spectra of enantiopure and racemic single crystals as well as powders of 2,2,5,5-tetramethylpyrrolidine-3-carboxamide-1-oxyl have been observed. Possible reasons of this fact are proposed. Chirality, 2006. © 2006 Wiley-Liss, Inc. [source] ESR and FTIR studies on electron beam-irradiated low-density polyethylene blendsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007Z. I. Ali Abstract Low-density polyethylene (LDPE) composites modified with a resin based on ethylene/methacrylic acid copolymer (surlyn) and/or citric acid were electron beam-irradiated and investigated by electron spin resonance (ESR) at room temperature. ESR studies were carried out directly after irradiation and after various time intervals up to 72 h postirradiation. The irradiated samples showed the ESR spectrum of seven lines that was assigned to the formation of allyl radical. The nature and yield of the allyl radical of the different LDPE samples were analyzed as a function of time after irradiation. Also, the radical concentration, decay, decay rate, and kinetics of radical decay were evaluated. Fourier transform infrared (FTIR) analysis at a series of different temperatures upon cooling from room temperature to ,175°C and the reverse heating to +125°C was also carried out. The structural changes while cooling and heating of LDPE samples were investigated using FTIR spectrometry. The results showed that cooling of unirradiated LDPE samples to ,175°C results in a decrease of the intensities of IR bands. However, heating the samples from ,175°C up to +125°C led to a consequence increase in the intensities of the IR bands. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3461,3469, 2007 [source] Radical polymerization of n -butyl methacrylate initiated by stibonium ylideJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2007Sumita Srivastava Abstract 1,2,3,4-Tetraphenylcyclopentadiene triphenyl stibonium ylide initiated radical polymerization of n -butyl methacrylate (n -BMA) in dioxane at (60 ± 0.2)°C for 90 min under nitrogen atmosphere has been carried out. The system follows nonideal kinetics, i.e., Rp , [ylide]0.2 [n -BMA]1.8. The value of k/kt and overall energy of activation have been computed as 0.133 × 10,2 L mol,1 s,1, 33 kJ/mol, respectively. The FTIR spectrum shows a band at 1745 cm,1 due to acrylate group of n -BMA. The 1H NMR spectrum shows a peak of two magnetically equivalent protons of methylene group at 2.1 , ppm. The DSC curve shows glass transition temperature (Tg) as 41°C. The presence of six hyperfine lines in ESR spectrum indicates that the system follows free radical polymerization and the initiation is brought about by phenyl radical. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2457,2463, 2007 [source] Reduction of ciclosporin and tacrolimus nephrotoxicity by plant polyphenolsJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 11 2006Zhi Zhong The immunosuppressants ciclosporin (cyclosporin A, CsA) and tacrolimus can cause severe nephrotoxicity. Since CsA increases free radical formation, this study investigated whether an extract from Camellia sinensis, which contains several polyphenolic free radical scavengers, could prevent nephrotoxicity caused by CsA and tacrolimus. Rats were fed powdered diet containing polyphenolic extract (0-0.1%) starting 3 days before CsA or tacrolimus. Free radicals were trapped with ,-(4-pyridyl-1-oxide)- N - tert -butylnitrone (POBN) and measured using an electron spin resonance spectrometer. Both CsA and tacrolimus decreased glomerular filtration rates (GFR) and caused tubular atrophy, vacuolization and calcification and arteriolar hyalinosis, effects that were blunted by treatment with dietary polyphenols. Moreover, CsA and tacrolimus increased POBN/radical adducts in urine nearly 3.5 fold. Hydroxyl radicals attack dimethyl sulfoxide (DMSO) to produce a methyl radical fragment. Administration of CsA or tacrolimus with 12C-DMSO produced a 6-line spectrum, while CsA or tacrolimus given with 13C-DMSO produced a 12-line ESR spectrum, confirming formation of hydroxyl radicals. 4-Hydroxynonenal (4-HNE), a product of lipid peroxidation, accumulated in proximal and distal tubules after CsA or tacrolimus treatment. ESR changes and 4-HNE formation were largely blocked by polyphenols. Taken together, these results demonstrate that both CsA and tacrolimus stimulate free radical production in the kidney, most likely in tubular cells, and that polyphenols minimize nephrotoxicity by scavenging free radicals. [source] Investigation of a CuII,Poly(, -Glutamic Acid) Complex in Aqueous Solution and its Insulin-Mimetic ActivityMACROMOLECULAR BIOSCIENCE, Issue 4 2007Subarna Karmaker Abstract The complexation between cupric ions (CuII) and poly(, -glutamic acid) (, -PGA) in aqueous solutions (pH 3,11) has been studied by UV-visible absorption and electron spin resonance (ESR) techniques. Formation of the CuII - , -PGA complex is confirmed by the observation of the blue shift of the absorption band in the visible region, anisotropic line shapes in the ESR spectrum at room temperature, and a computer simulation of the visible absorption spectrum of the complex. The structure of the CuII - , -PGA complex, depending on the pH, has been determined. The in vitro insulin-mimetic activity of the CuII - , -PGA complex is examined by determining both inhibition of free fatty acid release and glucose uptake in isolated rat adipocytes treated with epinephrine, in which the concentration of the CuII - , -PGA complex for 50% inhibition of free fatty acid release is very similar to that of CuSO4. However, it is significantly lower than that of a previously reported insulin-mimetic bis(3-hydroxypicolinato)copper(II), [Cu(3hpic)2], complex. [source] Nitroxide spin probe study of probe size, hydrogen bonding and polymer matrix rigidity effects on poly(acrylic acid)/poly(ethylene oxide) complexesMAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2003Li Tan Abstract An electron spin resonance (ESR) spin probe study was performed on 1 : 1 by weight poly(acrylic acid) (PAA)/poly(ethylene oxide) (PEO) complex over the 100,450 K temperature range with a series of tetramethylpiperidyloxy-based spin probes. Measurements of the parameters T5mT, Ta and Td demonstrated the effects of probe size and the strength of hydrogen bonding. The probes in the series Tempone, Tempo, Tempol and Tamine (respectively 4-oxo-, unsubstituted, 4-hydroxy- and 4-amino-2,2,6,6,-tetramethylpiperidine -1-oxyl) displayed noticeable increases in the hydrogen-bonding effect, as indicated by Ta and Td. These increases correlated with increasing hydrogen bond acceptor strength. On the other hand, as the probe size became larger, T5mT gradually increased due to the free volume decrease. These effects were analyzed using the established theoretical relationship of T5mT to probe volume expressed by f. Meanwhile, in order to investigate the effect of polymer matrix rigidity, a similar study was performed with a nitroxide spin probe, 2,2,6,6-tetramethyl-1-piperidine-1-oxyl (Tempo), on PAA/PEO complexes of different weight compositions. The quantitative fast motion fraction in the composite ESR spectrum was calculated. The influence of changes in the composition of PAA on the molecular mobility was characterized by changes of the spectral parameters and ,c. The molecular mobility was shown to diminish with increasing content of PAA in PAA/PEO blends duo to the restriction of the polymer matrix rigidity increase. Copyright © 2003 John Wiley & Sons, Ltd. [source]
| | ||||||||||||||||||